Nuclear receptors are a family of ligand-regulated transcription factors that are activated by one of two main classes of ligands, steroid hormones – such as estrogen and progesterone – and non steroidal ligands, including retinoic acid and thyroid hormone.
A number of nuclear receptors have no known endogenous ligands and are considered orphan receptors, these include FXR, LXR, and PPAR. Instead these receptors bind with low affinity to a number of metabolic intermediates and may function as metabolic sensors.
A unique property of nuclear receptors, that differentiates them from other classes of receptors, is their ability to directly interact with and regulate transcription of genes. This allows then to control a wide variety of biological processes, such as the cellular response to sex steroids, vitamin D3, adrenal steroids and other metabolic ligands. Nuclear receptors are important drug targets in reproductive, metabolic and proliferative diseases.